The SMR process is mainly based on the reforming reaction of light hydrocarbons such as methane that yields to a mixture of hydrogen (H2) and carbon monoxide (CO) in the presence of water vapor. The reaction is endothermic and slow and requires additional heat input, as well as a catalyst to occur. The SMR reactor usually comprises several tubes placed in a furnace, said tubes filled with catalyst pellets and fed with the process gas mixture of methane and steam.
Several types of furnace designs are encountered industry wide. The top-fired technology is one of the most referenced designs and it is proposed by several technology providers. Top-fired furnaces are typically made of a refractory lined firebox containing several rows of catalyst containing tubes. Roof burners are placed in rows in-between the tube rows and the combustion products out of the burners are usually blown vertically downwards, so that the tube rows face the flames in their upper part. A flue gases exhaust collector is usually provided at the furnace floor level.
The main objective of the furnace design, often also called firebox design, is to maximize the heat transferred from the burner flames to the tubes while respecting a tube maximal operating temperature constraint that is a function of the tube mechanical load (mainly feed gas pressure), the mechanical properties of the alloys used for the tubes and the desired lifetime of the tubes. Indeed, any intensification of the heat transferred to the tubes has a direct positive impact, either on the furnace productivity or on the compactness of the firebox which is valuable in terms of capital expenditures. However, intensification of the heat duty is usually implying higher tube skin temperature levels that reduce tube lifetime or require more resistant alloys, which are much more expensive.
The temperature profiles of the catalyst tubes are therefore a critical element of the furnace design and operation, at the focal point of the compromise between performance and durability. The typical vertical profiles for tube heat flux and temperature is plotted in FIG. 2 in circumferential average. The heat flux profile clearly highlights that the feed inlet (upper) part of the tube is a zone for heat transfer. Indeed, several factors favor the maximization of the heat flux:                Vicinity of the burners and feed inlet point, implying a maximum temperature difference between the load (tubes) and the heat release source (burners)        Highest reaction rates and thus heat sink that pull tube temperatures down        
This underlines the superiority of the top-fired design compared to others with regard to heat transfer efficiency.
The stiffer the heat flux and temperature profile is in the upper part of the tube, the higher is the heat duty to the tube at same (creep resistance) design temperature, and thus the higher the process gas flow rate capacity per tube at the same conversion rate. The actual top-fired design to increase the heat transfer in the upper part of the furnace is however limited to the capacity of gaseous flames produced by conventional burners used in the furnaces to transfer the chemical energy to the tubes from the radiation of hot gases. Indeed, several phenomena limit the ability of conventional down-fired burners:                High nitrogen oxide (NOx) levels are heavily penalizing short flames burner selection for environmental reasons, whereas a typical means to reduce thermal NOx emissions is to dilute the flame with burnt gases, or to stage the fuel injections so that the flame maximum temperature is reduced below 1000° C. As a result, the flame's capability to transfer heat in the upper part of the furnace and, therefore, the heat provided for the reaction is reduced. This constraint is a typical compromise between longer and colder flames and shorter, less NOx efficient ones.        The physics of radiation heat transfer between gaseous media and walls is intrinsically less efficient than the ones between wall surfaces of different temperatures. The 1 m characteristic dimension volume of hot gases has typical net emission largely below the one of high emissivity solid surface heated at the same temperature.        
Furthermore, in top-fired reformers the necessary heat for the endothermic reaction to occur is provided by burners located between the tubes. Additional burners at the furnace side, along the walls of the furnace are only heating one tube row on one side and the refractory wall on the other side. The burners in the middle of the firebox are heating two tube rows on both sides of the burner row. Therefore, the required power of the side burners is smaller (˜52% including heat losses at side wall) than the one in the center of the furnace. Reducing the power injected at the side burner rows, while keeping the stoichiometry constant, implies to reduce the air and fuel flow rates.
The fluid mechanism and jets theory will define the typical flow arrangement within a top-fired firebox, meaning the side burners hot burnt gases jets aspiration towards the middle center of the firebox. The jet flame entrains part of the surrounding flue gas, creating a depression, and consequently a flue gas recirculation. Therefore the burners located along the walls are submitted to a smaller recirculation (i.e. depression) on the wall side than on the furnace side, due to the presence of the next burner row. If the lower power or flow rates along the side walls yields to a lower velocity, this will reinforce the bending effect of the side flames to the center, due to the weaker momentum of the side jets, as illustrated in FIG. 3.
In US 2007/0099141 A1 a method and furnace for generating straightened flames in a furnace are proposed, wherein an oxidant is introduced into a plurality of oxidant conduits. Each of the oxidant conduits has an outlet in fluid communication with a furnace interior proximate a first interior end of the furnace. The first interior end of the furnace has a horizontally projected area. The oxidant conduit outlets define a combined horizontally projected turbulent free jet area at 30% of the average distance from the first interior end of the furnace to a second interior end of the furnace provided opposite the first interior end.
Document US 2007/0128091 discusses a furnace chamber surrounded by a circumferential furnace wall, in which a plurality of burners disposed essentially in one plane, with burner exit direction directed downward and a plurality of reaction tubes disposed essentially vertically and parallel to one another are disposed, whereby the reaction tubes are heated from the outside by means of firing burners. It is intended to improve the heat distribution and the entire heat transfer. This is achieved by disposing at least the outer burners in the region of the furnace wall with a burner exit direction which is inclined relative to the vertical, leading away from the center of the furnace.
Document EP 2 369 229 A2 describes a reformer and a method for operating this reformer including the combustion of fuel in a combustion region of an up-fired or down-fired reformer, wherein at least one of the burners is a wall-bound burner forming a nonuniform injection. The non-uniform injection properties generate a heat profile providing a first heat density proximal to a wall and a second heat density distal from the wall, wherein the second heat density is greater than the first heat density. The non-uniform injection properties are formed by selecting an angle of one or more injectors, a flow rate of one or more injectors, an amount and/or location of oxidant injectors, an amount and/or location of fuel injectors, and combinations thereof.
The article “Fluegas flow patterns in top-fired steam reforming furnaces” of W. Cotton, published in 2003 by Johnson Matthey, teaches that reformers comprising outer burners firing with a rate of 70% compared to the inner burners and an outer lane between the tubes and the furnace side that has 70% of the width of the inner lanes between two tube rows reduce the recirculating problem. According to the article it is also possible to operate with 100% rated outer burners firing into an outer lane having the same width as the inner lanes without any bending of the flames to the center of the furnace.
All proposed solutions have in common, that they do not enable a furnace design, which provides outer burners with only the required amount of power. As presented e.g. in the cited article “Fluegas flow patterns in top-fired steam reforming furnaces”, the burner power rate is not reduced to the calculated value of about 52%. Therefore, the known solutions avoid the flame bending to the furnace's center but do not prevent an overheating of the catalyst containing tubes situated close to the furnace walls. Such an overheating leads to unwanted side reactions and an irreversible damage of the catalyst.
Therefore, it is the object of the present invention to propose a furnace and a method to operate this furnace which will avoid the bending effect of the side flames to the center as well as the problem of overheating the tubes close to the walls of the furnace.